1. Field of the Invention
The invention relates to a An active electronic filter circuit, comprising
at least one filter input, PA1 at least one amplifier element having at least one amplifier input and for at least one of the filter inputs and at least one of the amplifier inputs a network which defines the filter characteristic and which comprises an input impedance arrangement for connecting the filter input to the amplifier input.
2. Description of Related Art
From U.S. Pat. No. 4,509,019 a tunable high-pass or low-pass filter is known, which filter includes a push-pull amplifier having electronically controllable elements in either the two input branches or the two feedback branches (or both in the input branches and the feedback branches). Each of the other branches includes a reactance element. This arrangement serves to attain a low distortion, which allows a simple integration on a semiconductor body using a non-linear element, for example a MOS transistor, as voltage-controlled element.
It has been found that this known filter circuit has the drawback that the time constant attainable with such a filter circuit is only adjustable over a comparatively small control range because the resistance values of the MOS transistors used as controllable elements can be varied only over a comparatively small control range.
From U.S. Pat. No. 4,780,690 a filter arrangement is known whose time constant should be adjustable over a comparatively large range. This filter arrangement comprises a push-pull amplifier having an inverting input, a non-inverting input, an inverting output, a non-inverting output, a first input terminal and a second input terminal, a first feedback circuit including a first capacitor and arranged between the non-inverting output and the inverting input, and a second feedback circuit including a second capacitor substantially identical to the first capacitor and arranged between the inverting output and the non-inverting input. This known filter arrangement further comprises a transconductance circuit with variable transconductance, which has two inputs connected to the input terminals of the filter arrangement, as well as two outputs respectively connected to the inverting input and the non-inverting input of the amplifier. The transconductance circuit is a voltage-controlled current source which converts a balanced input voltage into a balanced output current. The transconductance dictates the proportionality factor between the output current and the input voltage. Generally, this transconductance can be varied over a comparatively large range, as a result of which the time constant of the known filter arrangement should also be variable over a comparatively large range.
The transconductance circuit of the filter arrangement in accordance with U.S. Pat. No. 4,780,690 includes several active elements which each have a control input and a first and a second input terminal. The control inputs of at least two of said active elements are connected to the input terminals of said filter arrangement. The first output terminals of said at least two active elements are connected directly to one another and the second output terminals of these two active elements are connected to said inverting amplifier input and said non-inverting amplifier input, respectively.
The filter arrangement known from U.S. Pat. No. 4,780,690 further comprises a load circuit for loading the transconductance circuit. This load circuit comprises a first and a second current source transistor whose interconnected base electrodes are connected to a node between two resistors via a diode, which resistors are arranged between the outputs of the transconductance circuit.
From U.S. Pat. No. 4,780,690 it is further known to form a transconductance circuit by means of MOS transistors. This known transconductance circuit is also assumed to have a comparatively large linear voltage range.